Generally, in various systems using light such as image observation, a sensor, security, laser radar and the like, the technique of detecting desired light is a fundamental and important element significantly influencing their performance. In particular, the needs for the detection technique with high speed and high sensitivity are high.
For example, there has been traditionally and widely used an optical inspection method in which an organism is irradiated with light and then the transmitted light, reflected light or scattered light is detected to extract in vivo information from such detected signal light. In particular, the optical imaging technique, two-dimensionally scanning a region to be inspected with light and displaying information of obtained signal light as an image, contributes greatly to the field of medicine.
Moreover, in accordance with the development of laser technique, there are recently being active the biological or medical studies using laser scanning-type imaging such as a laser scanning-type microscope, a laser scanning-type microscopic endoscope or the like. In particular, the laser scanning-type fluorescent imaging method enables serial observation of living cells at a high signal-to-noise ratio, thus being an essential tool for biological or medical studies. Among such methods, the multiphoton fluorescent imaging method using multiphoton excitation when obtaining fluorescence enables observation of deep portion of an organism, thus drawing attention as a new fluorescent imaging method (see Non-Patent Documents 1, 2, for example).
Moreover, there is getting active the research development of a laser scanning-type imaging method using nonlinear optical effects in organisms such as Coherent anti-Stokes Raman Scattering (CARS) imaging (see Non-Patent Document 3, for example), high-frequency generating imaging (see Non-Patent Document 4) or the like. Since the laser scanning imaging method using the nonlinear optical effects does not require dyeing of a sample to be observed with fluorescent substances such as fluorescent protein, fluorescent dye or the like, the method has an advantage, as compared with the fluorescent imaging method, that the true state of organisms can be observed.
In this connection, in the optical imaging for organisms including the laser scanning-type imaging, optical signals obtained from an organism sample are usually weak due to influences by light scattering effects and light absorbing effects in the organism sample, or the like. Particularly in the imaging method using nonlinear optical effects such as the multiphoton fluorescent imaging, the CARS imaging or the like, the conversion efficiency from excitation light to signal light is essentially low, and thus the optical signals obtained from an organism are significantly weak. Therefore, it is difficult to obtain a clear image.
As a method of solving this problem, it is conceivable that the intensity of excitation light with which an organism is irradiated is rendered to be higher. However, when an organism is irradiated with light having excessively high intensity, it could be possible to damage the organism. Thus, the intensity of excitation light has an upper limit. Therefore, it is difficult to obtain a clear image in lots of cases.
Then, it is general that a low-noise high-sensitivity photodetector is used to obtain a clearer image.
As the currently-used typical photodetection device, there can be mentioned PMT (Photo multiplier tube), APD (Avalanche photo diode) and PD (Photo diode). The PMT and the APD perform electron multiplying in the detection device, which enables high-sensitive photodetection. On the other hand, the PD does not have an electron multiplying function in the detection device and thus signals are usually amplified with the use of an electric amplifier, although it achieves a very high response speed. That is, any devices of PMT, APD and PD perform signal amplification in an electric domain to improve the sensitivity.
Moreover, there can be mentioned, as the typical two-dimensional photodetectors, CCD (Charged coupled device), CMOS (complementary metal Oxide semiconductor), EM-CCD (Electron multiplying-CCD), EB-CCD (Electron bombardment-CCD) and I-CCD (Intensified-CCD). When weak light is detected using the CCD or the CMOS, it is necessary, like the PD, to dispose an electric amplifier in a subsequent part so as to improve the sensitivity. The EM-CCD and the EB-CCD have an electron multiplying function in the detection device, like the APD, and achieve the higher sensitivity. The I-CCD has a configuration in which an Image intensifier (hereinafter referred to as I.I.) is disposed before the CCD. In the I.I, incident light signals are converted to electrical signals once, and electron multiplying is performed in a MCP (Micro channel plate) embedded in the I.I., thereafter the multiplied electrons are rendered to collide with a fluorescent plate so that the multiplied electrical signals are converted to light again. That is, the I-CCD also performs signal multiplying in an electric domain, achieving high-sensitive photodetection.
However, the low-noise and high-sensitivity photodetector is of particular and very expensive. Moreover, in the above conventional photodetection technique using signal amplification in an electric domain, the volume of noises generated from a photodetector and accumulated time of signal light by the photodetector have the inverse relationship, that is, the trade-off relationship therebetween, which requires sufficiently long accumulated time to obtain low-noise detection signals. As a result, sufficiently long accumulated time is needed to obtain a clear image after detecting weak signal light, thus extending time for obtaining an image. Therefore, real-time properties lack for needs to properly observe an organism varying with time, and it could be possible to inhibit basic needs by optical imaging users. Therefore, it is unavoidable in the present situation that either of detection speed or detection sensitivity is sacrificed to perform photodetection.
It is noted that such problem occurs not only in optical imaging methods but also in optical measuring methods such as the flow site meter optically analyzing fine particles in fluid, the fluorescence correlation spectroscopy (FCS) optically analyzing motion of fluorescently-labeled biomolecules in solution, the surface plasmon resonance method (SPR) optically analyzing the state of connection among biologically-relevant molecules fixed on the surface of a solid substrate and the latex photometric immunoassay (LPIA) optically analyzing the state of connection among biologically-relevant molecules in solution and the fluoroimmunoassay (FIA) detecting immune response in solution based on the existence or nonexistence of fluorescent label.
On the other hand, it has been conventionally active in development of the optical amplifier using optical fibers as the light transmission means mainly in the field of long-distance optical communication. As compared with the electric amplifier, the optical amplifier is capable of very high-speed wide-bandwidth operation and has properties, depending on the configuration, capable of low-noise and high-gain optical amplification. Such an optical amplifier is disposed before a high-speed photoelectric conversion device, thereby high-speed and high-sensitive photodetection can be expected (see Patent Document 1, for example).
The summary of Patent Document 1 is that optical irradiation is conducted by one of two optical fibers disposed at a given angle and the other performs photodetection, thereby the axial resolution in a depth direction of an object to be observed is improved.
Moreover, Amplified spontaneous emission (ASE) is generated from the optical amplifier, which is a dominant cause of noises in photodetection using the optical amplifier. Therefore, for achieving high-speed and high-sensitive photodetection using the optical amplifier, it is necessary that the optical amplifier is of low noise. With respect to the optical amplifier, as the light intensity of input signals becomes higher, the signal to noise ratio (hereinafter referred to as the SNR) after optical amplification is improved and thus the photodetection sensitivity is improved.
Moreover, a low-noise optical amplifier used in the field of long-distance optical communication is constituted by a single-mode optical fiber. The reasons why such a constitution by single-mode optical fiber is used are because the consistency with a transmission path is excellent and noises of the optical amplifier increase in proportion to a transmission mode of a gain fiber constituting the optical amplifier. Thus, in the field of long-distance optical communication, the use of a single-mode optical fiber as a gain fiber contributes greatly to low-noise properties of the optical amplifier.
However, even such a low-noise optical amplifier for long-distance optical communication constituted by a single-mode optical fiber has the following problem when used before a photoelectric conversion device. That is, it is often the case that signal light detected in the fields of organism observation, a sensor, security, laser radar and the like is scattered light or light with distorted wavefront. Such light has significantly low coupling efficiency to a single-mode optical fiber, and the light signal which can be taken in the optical amplifier is weak signals being of limited one portion of the whole. Therefore, the SNR is deteriorated and thus the high light-receiving sensitivity cannot be obtained.
On the other hand, it is conceivable that a multimode optical fiber amplifier having a large core diameter is disposed just before the photoelectric conversion device since it collects scattered light and light with distorted wavefront with high efficiency. However, since optical noises generated in the optical amplifier increase in accordance with the increase of the number of spatial modes resulted by increasing the core diameter, the SNR deteriorates in this case also and thus the high light-receiving sensitivity cannot be obtained.
Although the above is described with an example of a case in which light is detected, it is conceivable that cases in which electromagnetic waves other than light such as millimeter waves, microwaves and the like are detected also have the same problem.
On the other hand, there is conventionally known the organism tomographic image measuring technique using light referred as Optical coherence tomography (OCT) (see Non-Patent Document 5, for example). The OCT technique makes it possible to measure a tomographic image of an organism at a depth position of 1 mm to 2 mm, at resolution of about 1 μm to 10 μm.
The OCT technique is divided mainly to three methods of Time-domain (TD) OCT, Frequency-domain (FD) OCT (see Non-Patent Document 6, for example) and Swept source (SS) OCT (see Non-Patent Document 7, for example). Among them, the SSOCT using light whose wavelength varies with time enables measurement of an organism tomographic image at highest-speed and with highest-sensitivity, and the technical development thereof is being actively advanced.
However, the penetration depth of tomographic images obtained by OCT is presently about only 1 to 2 mm, and thus the performance is insufficient in diagnosis of penetration of cancer which is essential for early detection thereof. Thus, the applied range is limited.
The reason why the penetration depth by OCT is not improved is because optical signals returning from the deep portion of the organism to the surface thereof are weak due to light scattering effects or light absorbing effects in the organism and thus the signals from the deep portion are buried in noises such as shot noises, thermal noises, quantization noises and the like which are generated in the detection process. Particularly in the case of SSOCT, quantization noises (quantization errors) in an analog-digital converter (ADC) are the cause of limiting the penetration depth.
That is, in the SSOCT, analog signals after photoelectric conversion are of high frequency in a deep portion of an organism and of low frequency in a shallow portion thereof, and the signal from a shallow portion of the organism usually has higher intensity by several orders of magnitude than that from a deep portion thereof. Therefore, when such a signal is quantized by the ADC, high-frequency components with small amplitude having information from the deep portion of the organism are buried in quantization noises and information from the deep portion cannot be taken out even when an ADC having a relatively wide dynamic range of 14 bits is used, for example.    Patent Document 1: U.S. Pat. No. 6,423,956    Non Patent Document 1: W. Denk et al., Science 248, 73 (1990)    Non Patent Document 2: J. Jung and M. J. Schnitzer, Opt. Lett. 28, 902 (2003)    Non Patent Document 3: A. Zumbusch et al., Phys. Rev. Lett. 82, 4142 (1999)    Non Patent Document 4: I. Freund et al., Biophys. J. 50, 693 (1986)    Non Patent Document 5: D. Huang et al., Science 254, 1178 (1991)    Non Patent Document 6: R. Leitgeb et al., Opt. Lett. 25, 820 (2000)    Non Patent Document 7: S. R. Chinn et al., Opt. Lett. 22, 340 (1997)